The present invention relates to a tachometer system, and more particularly relates to a system for detecting the rotational speed of an internal combustion engine from a connection to a grounded portion of a vehicle containing the engine.
Tachometer systems for measuring the rotational speed of an engine conventionally attach to a spark plug wire of the engine. The ignition signal present on the spark plug wire is inductively or capacitively coupled to a direct pick-up tachometer, and the rotational speed of the engine, typically in revolutions per minute (rpm), is derived from the frequency of ignition spark pulses in the ignition signal. However, many engines have inaccessible spark plug wires or no spark plug wires at all, making direct pick-up tachometers difficult or impossible to use.
Radio frequency (RF) antenna tachometers have been developed to address some of the problems associated with direct pick-up tachometers. RF tachometers detect radiated signals from the ignition system of the engine to obtain the ignition spark pulses and derive the rotational speed of the engine therefrom. In many engines, RF emissions from the ignition system are faint, low-power signals, forcing the RF antenna of the tachometer to be highly sensitive in order to detect ignition signals. Differences between ignition systems for different types of engines and the associated levels of radiated ignition signals make it quite difficult to filter out RF emissions from adjacent vehicles while operating an RF antenna tachometer. This is especially true in the service and repair station context, where adjacent cars under service are relatively close by. Moreover, an open spark plug wire, such as a cracked conductor, may cause high voltage arcs that can overpower a nearby RF antenna tachometer system.
Therefore, there is a need for a tachometer system that reliably measures the rotational speed of an engine while overcoming the problems of direct pick-up and RF antenna tachometers.